In recent years, synthesis and property evaluation of nanoparticles having any properties of magnetic properties, luminescence properties, and plasmon absorption properties have been intensively studied. Nanoparticles that exhibit various sizes, compositions, polarities and solubilities, surface conditions, and the like have been developed, and, in particular, in terms of biochemical application, semiconductor nanoparticles and metal nanoparticles have received attention due to their distinctive optical properties. In particular, organic fluorescent dyes and the like have been commonly used as a detection label in existing biotools, but they have problems, for example, with luminescence properties, stability, and cost. Thus, recently, studies on the use of semiconductor nanoparticles having high luminescence and high stability as a detection label have received attention (see WO 00/017642).
To use nanoparticles in biochemical applications, it is necessary that the nanoparticles have excellent surface hydrophilicity and biocompatibility and that the nanoparticle properties be retained even in an aqueous solution. For the conventional method for water-solubilizing nanoparticles using a low molecular-weight solubilizer (see Dubertret, B. et al., SCIENCE, 2002, 298, 1759-1762), various problems have been pointed out: for example, hydrophilic functional groups introduced onto a nanoparticle surface are removed, or functional groups having an electric charge affect the nanoparticle surface to decrease the water dispersion stability, light stability, and the like of nanoparticles (see Yu, W. W. et al., J. AM. CHEM. SOC., 2007, 129, 2871-2879). There is also a method in which nanoparticles are encapsulated in microspheres, liposomes, hydrogels, or the like, but the development of its application is limited because of the too large size (see GaO, X. et al., J. Phys. Chem. B., 2003, 107, 11575-11578).
As a means for solving the above-mentioned problems, the method of coating a nanoparticle with polymers is known (see JP 2006-213592 A, JP 2008-540726 W, Anderson, R. E. et al., ACS NANO, 2008, 2, 1341-1352 and Andrew, M. S. et al., J. AM. CHEM. SOC., 2008, 130, 11278-11279). For example, by using an amphiphilic polymer having a side chain comprising a hydrophilic carboxyl group and a hydrophobic lipid-soluble compound, stable binding through interaction with a hydrophobic surface of nanoparticles is achieved, and the hydrophilic group can be used for water-solubilization of the nanoparticles and as a site for introducing biomolecules (see Anderson, R. E. et al., ACS NANO, 2008, 2, 1341-1352). On the other hand, the case has also been reported where, a single nanoparticle is coated compactly with a plurality of polymers to improve the water dispersion stability, light stability, and the like using, in place of the hydrophobic lipid-soluble compound, ligand exchange reaction between a nanoparticle surface and vinyl-based water-soluble polymers into the side chain of which a thiol group and an amino group are introduced (see Andrew, M. S. et al., J. AM. CHEM. SOC., 2008, 130, 11278-11279). However, in any of these cases, a plurality of polymers are associated with one nanoparticle, and thus it has been difficult to improve total emission intensity and the like of nanoparticles by accumulating a number of nanoparticles on one surface ligand by uniform complexation of a plurality of nanoparticles and polymers.
It could therefore be helpful to provide a water-soluble nanoparticle complex that allows for the use of nanoparticles for biochemical purposes.